After reading this, I got some Mothers R3 to clean the runner off the rear quarters. I didn't even know there was such a thing.  The Mothers worked amazingly well.  Spray it on, give it a minute, wipe with a towel.  If there is any rubber remaining, do it again.  It worked very well on the splash guards, making a black puddle on the floor as liquified the the tire rubber stuck to them.